1. Field of Invention
This invention relates generally to inspection systems and more specifically to inspection systems for detecting contraband items such as explosives.
2. Discussion of Related Art
Inspection systems that can detect contraband items are widely used at airports and other facilities where explosives or other contraband items could be particularly harmful. Many such inspection systems employ ionizing radiation to detect properties internal to items under inspection, thereby allowing the detection of contraband objects concealed within items under inspection.
At present, explosive detection systems for airports predominately use x-ray computed tomography (CT) to analyze the contents of checked airline luggage. CT systems detect explosive materials based on their x-ray absorption parameter values. Often a density map of the item under inspection that is computed from x-ray absorption information is used as an image of the item under inspection. Objects may be identified in the image based on their densities and the identified objects may be analyzed to identify contraband items. However, because the CT system works predominantly on density, innocuous objects having densities similar to the density of a contraband item may produce false alarms from the CT system.
Other detection technologies are known, even if not widely used at airports. These include coherent x-ray scattering, nuclear quadruple resonance (NQR), multi-view dual-energy x-ray, thermal neutron activation, fast neutron activation, and vapor detection.
It has also been proposed to use single view x-ray imaging systems in conjunction with CT based systems to improve the detection accuracy of the overall contraband detection system. The single view system may, for example, provide information about the effective atomic number of objects within an item under inspection. One approach to combining data from a single view and CT system is described in pending U.S. patent application Ser. No. 10/068,459 to Bijjani, et al., titled “METHOD AND APPARATUS FOR TRANSMITTING INFORMATION ABOUT A TARGET OBJECT BETWEEN A PRESCANNER AND A CT SCANNER,” filed Feb. 6, 2002, and published as 2003/147489 A1, now U.S. Pat. No. 6,816,571, which is hereby incorporated by reference.
Other systems have been proposed to combine imaging information with other information for improved detection of contraband items. For example, U.S. Pat. No. 5,642,393 to Krug et al., entitled “DETECTING CONTRABAND BY EMPLOYING INTERACTIVE MULTIPROBE TOMOGRAPHY”, which is hereby incorporated by reference in its entirety, describes an inspection system that uses more than one inspection technique for an item under inspection.
At present though, only CT-based systems have the required balance of specificity, spatial resolution, processing speed, system size, cost, and the ability to detect the necessary number of explosive types. CT system costs, however, are considered to be relatively high. In addition, CT systems, though certified, often generate false-alarms when the CT system incorrectly identifies that a piece of luggage or other item under inspection contains a contraband item or the CT system identifies that it could not, with a sufficient level of confidence, report that a particular item under inspection does not contain contraband. When a particular bag generates an alarm, significant activity is often required to clear the alarm bag. This activity may require manual inspection of the bag, which can be very costly.